The invention is directed to improvements in piezoelectric actuators, in particular for actuating control valves or injection valves in internal combustion engines in motor vehicles, having a piezoelectric actuator body, in particular in the form of a multilayer laminate comprised of layered plies of piezoelectric material and intervening metal or electrically conductive layers acting as electrodes, these electrode layers being contacted by electrically conductive common electrode leads, and the actuator body is surrounded by a metal module wall while maintaining an interstice that contains the common electrode leads.
One such piezoelectric actuator is disclosed for instance in German Patent Disclosure DE 196 50 900 A1 of Robert Bosch GmbH.
As is well known, piezoelectric actuators can for instance be used for injection valves of a vehicle motor and in brake systems with anti-lock and traction control systems.
Such injection valves equipped with piezoelectric actuators have an injection nozzle controlled by a tappetlike closure device. An operative face toward the nozzle is disposed on the tappet and is acted upon by the pressure of the fuel supplied to the nozzle; the pressure forces seek to urge the tappet in the opening direction of the closure device. The tappet protrudes with a plungerlike end, whose cross section is larger than the aforementioned operative face, into a control chamber. The pressure effective there seeks to urge the tappet in the closing direction of the closure device. The control chamber communicates with the fuel supply, which is at a high pressure, via an inlet throttle and with a fuel return line that has only low pressure, via an outlet valve that is throttled as a rule or is combined with an outlet throttle. When the outlet valve is closed, a high pressure prevails in the control chamber, by which the tappet is moved in the closing direction of the closure device, counter to the pressure on its operative face toward the nozzle, or is kept in the closing position. Upon opening of the outlet valve, the pressure in the control chamber drops; the magnitude of the drop in pressure is determined by the size of the inlet throttle and by the throttle resistance of the opened outlet valve, or the outlet throttle combined with it. As a result, the pressure in the control chamber decreases when the outlet valve is opened, in such a way that the tappet is moved in the opening direction of the closure device, or held in the open position, by the pressure forces that are operative on its operative face toward the nozzle.
In comparison with electromagnetically actuated injection valves, piezoelectric actuators can switch faster. However, in the design of a piezoelectric actuator, it must be noted that internal losses in the piezoelectric body of the actuator cause lost heat, which has to be dissipated so that the actuator will not overheat. Since the ceramic materials of the piezoelectric ceramic have poorer heat conductivity, the dissipation inside the actuator body, which substantially comprises ceramic material, is unfavorable, especially in long actuators, whose length is greater than their width.
Because of the electrodes that are located in the active part of the actuator body, its heat conductivity crosswise to the electrode layers is higher by a factor of three to five than at right angles to this, since the piezoelectric ceramic material is a poor heat conductor. Naturally, this factor depends on the geometric conditions, such as the thickness of the ceramic layers and the thickness of the electrode layers from the edge of the actuator body. Since as noted the heat conduction perpendicular to the electrode layers is poor, the heat takes the course through the electrodes of the active part of the actuator body to the common electrode leads located on the outside. Since these common electrode leads on the outside must not have any contact with the metal actuator base or the metal retaining plate in the region of the actuator head, the heat that occurs in the actuator body cannot be dissipated to these metal bodies unless further provisions are made. In particular, because of the high voltage, about 200 V, at the common electrode leads, special provisions are required for insulating the common electrode leads from the metal parts of the actuator module.
Cooling the actuator with a liquid coolant, such as fuel, water, motor oil and the like, which is theoretically possible, is unfavorable, first because of the risk of a short circuit from the water component that is contained both in the fuel and in motor oil, and second because the actuator module is more expensive because of complicated seals, which must preclude the coolant used from escaping from the actuator module, especially when the actuator becomes heated.
It is therefore the principal object of the invention to make a piezoelectric actuator in such a way that reliable cooling of the actuator body that heats up during operation is possible without using liquid coolant. Another object of the invention is the piezoelectric actuator can be installed in a simple way and does not require any special seals as in the case of cooling with liquid.
In an essential aspect of the invention, this object is attained in that the common electrode leads are designed in the form of a three-dimensionally structured surface or a three-dimensionally structured body i.e., the shape of the common electrode leads are not in a form of a conventional wire as usually used for electrodes, and the portion thereof contacting the actuator body has a lengthened portion or enlargement, protruding into the interstice, for dissipating heat from the actuator body.
In one embodiment, the lengthened portions of the common electrode leads are designed in the form of corrugated bands or foils that are parallel to the side walls of the actuator body.
In a second exemplary embodiment, to create a larger heat-conducting surface area, the common electrode leads located on the outside are lengthened by providing that the electrode leads in the interstice between the actuator body and the module wall are folded once or multiple times.
In a variant of the second embodiment, if a corrugated band is used for the common electrode leads, then the cooling area can also be increased by the design of the height of the corrugations.
In still another embodiment the conduction onward of the heat, given up by the enlarged or lengthened common electrode leads, to the module wall is preferably effected by means of a heat-conducting elastomer that fills up the interstice and envelops the lengthened common electrode leads.
Yet another embodiment involves a further increase in the cooling action of the enlarged or lengthened common electrode leads is accomplished by using an elastomer with electrically conductive particles and a high fill factor. In this case, the inside of the module wall must be electrically insulated, for instance by means of a layer of paint or oxide.
In still another embodiment for enlarging the heat-dissipating surface area of the common electrode leads, a lengthening of the lead wires can also be considered. Once again, the wire length and thus the heat-yielding surface area of the lead wire can be increased by folding it inside the interstice between the actuator body and the module wall.
In yet a further embodiment a combination of a lengthening, of the common electrodes leads optionally folded multiple times, of the common electrode leads, each with a lengthened lead wire, is equally possible.
In another still more favorable embodiment, one can introduce metal segments into the interstice, which are then welded or soldered to the electrode leads that are lengthened on both ends. These metal segments are preferably of copper and are glued to the module wall, which is provided with an electric insulation layer.
In a variant of the last embodiment, it is more economical but not quite so effective to use a metal wool or mesh, which is introduced between the common electrode leads and the module wall that is electrically insulated on its inside, and which is saturated with an elastic adhesive or an elastomer for fixation purposes.
In all the exemplary embodiments mentioned above of the piezoelectric actuator of the invention, the interstice can advantageously be filled with an elastomer, mixed with heat-conductive particles.
A piezoelectric actuator designed with the above characteristics utilizes the common electrode leads, located on the outside, which for the sake of good heat dissipation have been lengthened or enlarged, to dissipate the heat from the actuator. Thus solely solid bodies are utilized for heat dissipation, and hence no leakage problems of the kind that sometimes occur if the actuator is liquid-cooled can arise. By sheathing the actuator body with an elastomer or the like, no risk of short-circuiting from splashing water or other dirt occurs.
The piezoelectric actuator according to the invention is especially suited for use in common rail diesel injector systems and for PDE injector systems. Gasoline direct injection valves can also advantageously be made with a piezoelectric actuator of this kind.
The above and additional objects and advantages of the invention will be described in further detail below in conjunction with the drawings, which show several exemplary embodiments.